C 中对左值的要求

Question

What are rvalues, lvalues, xvalues, glvalues, and prvalues? 中对左值有相当广泛的描述，但这是针对 C++ 的，我认为其中很多不适用于 C。

我对左值的理解如下：

数组不是，但它们的下标值可能是，只要那些不是数组。例如：

int arr[3][2];
arr       // no --> int[3][2]
arr[1]    // no --> int[2]
arr[1][1] // yes --> int

结构可以直接或通过成员访问，例如 . 或 ->，只要它们不计算为数组即可。例如：

struct Member {int id};
Member member = {.id=2}, *member_ptr = &member;
member = {.id=3};    // ok
member_ptr = &member; // ok
member.id=3; // ok
member_ptr->id=4; // ok

address-of & 运算符不能。例如：
```
a = 7;
b = &a;
&b = &a; // invalid
```

* 运算符的值只要不引用数组即可。

int x[2][2] = {{1,2}, {3,4}};
*(x+1)=3;        // not ok, evaluates to x[2]
*(*(x+1)+1) = 3; // ok

除了 7=x 等更明显的情况外，我的理解中是否遗漏了主要项目？或者上面有什么不正确的项目？

Answer 1

C standard在6.3.2.1p1节中定义了一个左值如下：

An lvalue is an expression (with an object type other than void) that potentially designates an object; if an lvalue does not designate an object when it is evaluated, the behavior is undefined. When an object is said to have a particular type, the type is specified by the lvalue used to designate the object. A modifiable lvalue is an lvalue that does not have array type, does not have an incomplete type, does not have a const-qualified type, and if it is a structure or union, does not have any member (including, recursively, any member or element of all contained aggregates or unions) with a const-qualified type.

此定义包括数组，但数组不是可修改的左值。

间接运算符 * 的结果也是一个左值，因为它指向一个对象。如果结果对象是数组，这也适用。

数组下标运算符 [] 也产生左值，因为 x[y] 完全等同于 *(x + y).

成员访问运算符.和指向成员的指针运算符->的结果也是一个左值。

复合文字也是左值。例如，以下是有效的：

int *p = (int [3]){1,2,3};
p[0] = 4;

C 中对左值的要求

Requirements for lvalue in C

c

lvalue